1. Field of the Invention
This invention relates to the field of avionics, and, more particularly, to altimeters which may be readily exchanged without the need for in-flight recalibration.
2. Description of the Related Art
One of the most important pieces of aviation equipment is the altimeter, which calculates and displays the altitude of the aircraft. Current international aviation rules provide a preference to aircraft which are capable of determining their altitude within a predetermined accuracy, so there is a premium on precise measuring of actual altitude.
The basic method of determining altitude while an aircraft is in flight is by measuring the static air pressure on the exterior of the aircraft, and then calculating the altitude as a measure of height above sea level. Altitude measurement based on raw pressure data is not, however, a truly precise calculation.
That initial measurement is affected by many sources of static source error, including pressure measurement and conversion errors, line errors (leaks, etc.), barometric-correction errors, and others. Some sources of error (such as line errors) may be minimized (but not completely eliminated) by careful manufacturing procedures and quality assurance in aircraft manufacture. Other sources of error, such as pressure variations due to aircraft geometry (e.g., fit of doors near static ports, shape of static port, placement of static ports) are essentially constant over time, and may be factored out of pressure calculations to some degree by establishing a static source error correction (xe2x80x9cSSECxe2x80x9d). These latter types of error are collectively referred to as xe2x80x9cstatic source errorxe2x80x9d and may be derived from flight calibration measurements.
Static source error for a group of aircraft may be similar but will vary among individual aircraft. Each aircraft, therefore, has its own unique SSEC which is typically represented by a series of curves representing the SSEC for an individual aircraft and each altimeter, if more than one altimeter is used, at each of a range of aircraft flight speeds.
It is well-known in the art to provide SSEC, and the necessary calculation and monitoring of an aircraft""s SSEC have been incorporated in aviation regulations that have been in place for decades.
However, altimeters are mechanical devices that must periodically be replaced, and the replacement altimeters require recalibration when installed, because known altimeters do not have the ability to recall the SSEC used with a prior altimeter. This means that each installation of a new altimeter requires re-calibration, a process that is costly and time-consuming. In some applications, such as where the aircraft uses a redundant altimeter system such as described in my referenced co-pending application, some altimeters may have their SSEC stored in on-board flight computers, so that replacement of the altimeters is a simple matter. However, not all aircraft are so equipped, and even in aircraft with two redundant altimeters, having independent flight computers, systems which utilize the third backup altimeter disclosed in my referenced application do not have three flight computers for storing the SSEC of the backup altimeter.
There is accordingly a need in the art for an altimeter that does not require re-calculation of an aircraft""s SSEC each time that it is replaced. It would also be useful if such an altimeter could be employed within a redundant altimeter system such as described in my prior referenced application.
It is therefore an object of the present invention to provide an altimeter that retains the SSEC for an aircraft and is operable to pass on the SSEC to a new altimeter when the original is replaced.
In accordance with the invention, the inventive altimeter contains within it a means for storing the Static Source Error Correction (SSEC) for the altimeter. The altimeter also includes means for downloading the SSEC and for transferring it to a new altimeter which may then be used in place of the original without loss of the information built up over time or requiring re-calibration of the new altimeter based on newly performed tests, as well as means for uploading the SSEC from another altimeter.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.